docs: def addRelationship(self,
                          originItemId,
                          destinationItemId,
                          relationshipType):
          """
          Adds a relationship of a certain type between two items.
  
          Inputs:
             originItemId - The item ID of the origin item of the
                            relationship
             destinationItemId - The item ID of the destination item of the
                                 relationship.
             relationshipType - The type of relationship between the two
                                items. Must be defined in Relationship types.
          """
          url = "%s/addRelationship" % self.root
          params = {
              "originItemId" : originItemId,
              "destinationItemId": destinationItemId,
              "relationshipType" : relationshipType,
              "f" : "json"
          }
          return self._post(url=url,
                               param_dict=params,
                               securityHandler=self._securityHandler,
                               proxy_port=self._proxy_port,
                               proxy_url=self._proxy_url)